During the past decades there have been considerable developments within the fields of radiation therapy and tumor diagnosis. The performance of external beam radiation therapy accelerators, brachytherapy and other specialized radiation therapy equipment has improved rapidly. Significant developments have taken place in the quality and adaptability of the therapeutic radiation beams including new targets and filters, improved accelerators, increased flexibility in beam-shaping through new applicators, collimators, scanning systems and beam compensation techniques. Also improved dosimetric and geometric treatment verification methods have been introduced. Furthermore, advanced treatment planning systems capable of biological optimization of the intensity distribution of the delivered beams are now being available.
Intensity modulated radiation therapy (IMRT) is a fairly new method in which arbitrary dose distribution can be achieved in the target volume by modulating the intensity profiles of the incident therapeutic beams. Unlike conventional thereby employing uniform beams, IMRT can deliver almost arbitrarily shaped dose distributions that conform to the target volume, i.e. tumor volume, while sparing neighboring organs at risk and healthy tissues.
Various techniques for shaping intensity modulated beams (IMBs) have been developed, which typically can be categorized into static and dynamic fluence delivery techniques, respectively. In the static fluence techniques, the individual IMB is a result of a fixed intensity modulation by a static filter or similar structure to treat the target volume, which sometimes is called a step and shoot technique. In the dynamic fluence delivery technique, the target is encompassed by a continuously varied intensity profile.
The most common clinical fluence modulation technique for today is usage of multileaf collimators (MLCs). In this technique, the beam is collimated by multiple pairs of opposite tungsten leafs positioned perpendicular to the beam direction. The MLC is ideal for irregular static fields, but is marred by drawbacks in dynamic applications due to the increased treatment times and constraints on leaf position and mechanical limits in leaf velocity and acceleration. Helical tomotherapy is a special case of dynamic MLC where each leaf has only two positions, in or out. This tomotherapy technique resembles computerized tomography (CT) since the radiation source is rotated around the patient while the patient is moved axially through the field. A major drawback of dynamic MLC and the tomotherapy is lengthened total treatment time.
Intensity modulation can also be achieved by scanned beam therapy, in which a narrow, often Gaussian-shaped beam is scanned over the target. This modality has, so far, had somewhat restricted clinical use due to the increased cost of the required current systems. Furthermore, the beam resolution is limited for low energy electrons and photons. However, it is the state of the art technique for high energy electrons and photons as well as for light ions.
Static fluence modulation techniques for IMRT include usage of physical modulators, in which the whole target is encompassed simultaneously with a predetermined IMB profile. The IMB ca be achieved by intercepting the beam with a metal block of a thickness profile corresponding to the desired transmission profile. The simultaneous whole-filed irradiation results in higher monitor unit efficiency and less whole-body dose compared to sequential delivery of the dose segments. However, since traditional physical modulators require manual fabrication of different block shapes for each beam and manual exchange of these different blocks between beams, fixed physical modulators have to this date had limited clinical implementation.
Efforts have therefore been made ting to develop a more flexible physical modulator. A technique in which a machine automatically arranges metal cubes of two densities into certain patterns in order to provide a desired intensity modulator profile have been developed. Although this allows a more flexible modulator design, the total time of modulator profile exchange is still too large to be practically useful. In addition, the solid metal blocks inevitably imply restrictions.
Xu et al. [1] have presented a re-shapable modulator in which a mixture of tungsten powder, paraffin and silicon binder is shaped to a desired intensity modulator profile by a set of pistons. Still, the shaping is done outside the beam and therefore the modulator has no dynamic capabilities. In addition, a re-shaping of the modulator first requires shaping to uniform thickness before the target thickness distribution can be obtained, which prolongs the total re-shaping time The modulator presented by Xu et al., further has stability problems, implying that the shaped attenuating material may unintentionally deform during radiation therapy, especially when the modulator is in a non-horizontal position and/or is exposed to forces caused by rotation of the radiation gantry and the modulator.
Mark Carol [2] discloses an apparatus for conformal radiation therapy with a radiation beam having a pre-determined constant beam intensity that is spatially modulated across the tumor volume. The apparatus includes a housing having a plurality of compartments extending from the top to the bottom of the housing. Each such compartment has an inflatable balloon and is in contact with a pressurized reservoir containing mercury. When a particular balloon is deflated, mercury is pushed from the reservoir into the compartment associated with the balloon. Correspondingly, when a balloon is inflated, mercury is pushed from the associated compartment into the reservoir. The beam modulation is obtained by varying the amount of time each compartment is empty or fined with mercury.